1. Field of the Invention
The present invention relates to a composite sliding material suitable for bearing materials used in automobiles, agricultural machines, industrial machines and so on.
2. Brief Description of the Art
Composite sliding materials, in which a sintered copper alloy is bonded on a steel plate, have been hitherto used for bearings, bushings, washers, etc. in the fields of automobiles, agricultural machines, industrial machines and so on. The sintered copper alloy is typically a Cuxe2x80x94Snxe2x80x94Pb (lead bronze) alloy, which exhibits good sliding properties because the Cu alloy matrix supports a load and Pb improves anti-seizure property with a mating member.
The sintered copper alloy has improved anti-seizure property by virtue of additive Pb in bronze. However, it is preferable not to use Pb as far as possible since Pb adversely affects the environment. Thus, Bi is a possible alternative to Pb, but with regard to the anti-seizure property it is not expectable for Bi to have generally the same level effect. There is also a material which is of bronze containing graphite (a solid lubricant). However, it is inferior in mechanical properties because of lack of Pb as a low melting-point metal to be a liquid phase when sintering.
The present invention has been accomplished in view of the above background.
An object of the invention is to provide a copper system composite sliding material without Pb having excellent sliding properties equal to or superior to those of the lead-bronze system sintered copper alloy while having excellent mechanical strength.
The present inventors have found that, in the case where Bi and a solid lubricant are added into bronze, since Bi melts when sintering because Bi has a melting point as low as Pb, sintering property is improved and good anti-seizure property by virtue of the solid lubricant can be obtained, and that the coexisting solid lubricant and Bi prevent deterioration of mechanical strength of the bronze containing the solid lubricant and Bi, whereby the present invention has been achieved.
According to one aspect of the invention, there is provided a composite sliding material comprising a steel plate and a sintered copper alloy layer which is bonded to the steel plate, wherein the copper alloy comprises 1.5 to 15 mass % of Sn, 1.5 to 15 mass % of Bi, 1.5 to 20 volume % of a solid lubricant, and balance of Cu and incidental impurities, and wherein a volume ratio of Bi to the solid lubricant is in a range of 0.5 to 2.0.
According to another aspect of the invention, there is provided a composite sliding material comprising a steel plate and a sintered copper alloy layer which is bonded to the steel plate, wherein the copper alloy comprises 1.5 to 15 mass % of Sn, 0.03 to 1 mass % of P, 1.5 to 15 mass % of Bi, 1.5 to 20 volume % of a solid lubricant, and balance of Cu and incidental impurities, and wherein a volume ratio of Bi to the solid lubricant is in a range of 0.5 to 2.0.
Herein below, there will be provided reasons why the above components are contained in the copper alloy of the invention composite sliding material in the specific quantities, respectively.
(1) Sn: 1.5 to 15 Mass %
Sn strengthens the Cu matrix of the invention copper alloy. When the Sn amount is less than 1.5 mass %, the effect of strengthening the Cu matrix can not be obtained. When it exceeds 15 mass %, the Cu matrix becomes brittle because of much formation of a Cuxe2x80x94Sn compound.
(2) Bi: 1.5 to 15 Mass %
Bi becomes liquid when sintering and improves sintering property of the copper alloy. Further, Bi improves anti-seizure property and wear resistance of the copper alloy in an oil lubricating region, and also improves machinability together with the coexisting solid lubricant. When the Bi content is less than 1.5 mass %, the effects on anti-seizure property and wear resistance of the copper alloy can not be attained, and machinability of the copper alloy is deteriorated due to lack of an enough coexistence of Bi and the solid lubricant. When the Bi content exceeds 15 mass %, the copper alloy is deteriorated in strength.
(3) Solid Lubricant: 1.5 to 20 Volume %
The solid lubricant improves anti-seizure property and wear resistance of the copper alloy in the boundary lubrication state or the non-lubrication state. When the content of the solid lubricant is less than 1.5 volume %, the effects of improving anti-seizure property and wear resistance of the copper alloy can not be observed. When the content of the solid lubricant exceeds 20 volume %, the copper alloy is deteriorated in strength and machinability.
(4) Volume Ratio of Bi to a Solid Lubricant: 0.5 to 2.0
When the volume ratio of Bi to a solid lubricant (volume of Bi/volume of the solid lubricant) is in a range of 0.5 to 2.0, Bi and the solid lubricant coexist such that the solid lubricant is entrained in Bi, as shown FIG. 1. Thus the solid lubricant hardly leaves from the metal structure resulting in improved anti-seizure property and wear resistance. The most preferable volume ratio for coexistence of Bi with the solid lubricant is approximately 1. When the volume ratio of Bi to the solid lubricant is less than 0.5, the Bi effect is decreased, which is of restraining deterioration of the tensile strength of the copper alloy caused by the solid lubricant. When the ratio exceeds 2.0, the effect of the solid lubricant on anti-seizure property is deteriorated.
The solid lubricant may be one or more substances selected from the group consisting of graphite, BN, MoS2 and WS2. It may be of course any solid lubricant other than the above substances.
(5) P: 0.03 to 1 Mass %
P (phosphorous) improves the strength of the Cu matrix. When the P content is less than 0.03 mass %, it is impossible to improve the strength of the Cu matrix. When the P content exceeds 1 mass %, the Cu matrix becomes brittle.
According to one feature of the invention composite sliding material, the copper alloy further comprises not more than 40 mass % in total of one or more elements selected from the group consisting of Fe, Al, Zn, Mn, Co, Ni and Si. These elements dissolve in the Cu matrix to strengthen the Cu matrix. When the content of the element(s) exceeds 40 mass % in total, the hardness of the Cu matrix becomes too high resulting in deterioration of conformability.
According to another feature of the invention composite sliding material, a copper plating layer, having a thickness of not less than 2 xcexcm, exists between the steel plate and the sintered copper alloy layer. The copper plating layer improves the bonding strength between the sintered copper alloy layer and the steel plate. When the thickness is less than 2 xcexcm, such effect can not be obtained.
According to still another feature of the invention composite sliding material, the sintered copper alloy has a tensile strength of not less than 200 N/mm2 thereby restraining deterioration of fatigue resistance thereof.